Method of preparing molten magnesium alloy for casting



United States Patent METHOD OF PREPARING MOLTEN MAGNESIUM ALLOY FOR CASTING Keith Roberson, Midland County, and Elmer H. Mossner,

Saginaw, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application March 7, 1957 Serial No. 644,455

'4 Claims. (Cl. 75-168) The invention relates to methods of preparing molten magnesium-base alloy for casting. It more particularly relates ate a method of preparing a melt of magnesiumbase alloy .containing both zinc and zirconium.

Among the difliculties encountered in attempting to prepare for casting a melt of a magnesium-base alloy containing both zinc and zirconium as the principal alloying elements is that upon solidification undesirable segregation occurs of zinc-zirconium compound. The compound appears in the cast metal as a separate phase of the zinc-zirconium compound, Zn Zr So long as the size of the particles of the compound is small, e.g. having a maximum dimension of less than'% inch, no great disadvantage totheirpresence is found in the cast metallused as such orin'extrudes and forgings made fromthe cast metal.

undesirable segregation manifests itself particuiarlyzin. the process of continuous casting of large'billets 'or ingots. -;In.such operations the molten alloy is run into a cooled open ended mold in which the metal solidifies and .is withdrawncontinuously from the bottom of the mold -as rapidly as solidification occurs. solidification is;promoted usually by applying a coolant, such as water, to the cast metal as it emerges from the mold as well as by cooling the mold. An apparatus for and method of continuousv casting of magnesium alloy is disclosed in U.S.Patent No. 2,503,819.

"In view of the difficulty of preparing melts of zinc a-ndizirconium containing magnesium-base alloy which do not'yield undesirable segregation on solidification into cast metal, particularly in the continuous casting process of inakingingot, a considerable number of melts were prepared invarious .ways to discover a method capable of yielding-satisfactory segregate free cast ingot. Among the results of the preparation of these melts is the dis- .covery thabby following a particular sequence of steps of alloyingthe zinc and zirconium with the magnesium, treatmentroftheresulting melt before casting, and adhering to particular temperature schedules during these operations ingot can be cast from the melt without undesirable segregationof the zinc-zirconium compound. The invention then .consists of the improved method for preparing a melt of magnesium-base alloy containing zinc and zirconium herein fully described and particularly pointed out imthe claims.

in carrying out the invention, the requisite amount of zinc and zirconium are alloyed with magnesium, if not already present, as by melting down a batch of magnesium and adding thereto solid zinc and zirconium in a steel crucible using a suitable saline flux for protecting .ith hfiated metal from attack by the atmosphere. A suitable fiuxis one composed of 50 parts of magnesium chloride (MgC1 25 parts of potassium chloride (KCl), 20 partsofbarium chloride (BaCl and 5 parts of calcium fluoride (CaF Although zirconium can be added to the eha rge in the crucible as metallic zirconium better ,alloyingefficiency is achieved by the use of a reducible ripening: sal as or ex mpl zi c nium e chl ride,

preferably in 'admixturewith an equal weight-of potassium chloride. The alloying efiiciency of zirconium when introduced into the melt by reduction of the tetrachloride is from about 50 to percent. The efiiciency of alloying of the zinc is high being generally about 98 percent. The proportion of zinc to use may range from 1 to 9 percent of the weight of the alloy. Current commercial applications of the magnesium-base zinc and zirconium containing alloys generally employ about 4.8 to 6.5 percent of zinc, nominally 6 percent is preferred. The zirconium content is at least 0.1 percent and may be as high as 0.9 percent. A'preferred amount is in the range is eifected may range from about 12l0 to 1450 F., a preferred range is from 1350 to 1400 F. At the lower temperatures of the range alloying is slower. Agitating h m in Contact w t th zi onium ,pr ote allor n Once the magnesium is molten and the alloying constituents (zinc and zirconium) are alloyed in the melt,;i;t s treatment prior to and including the casting operation becomes critical, wehave found,-as regards obtaining cast metal adequately free from segregation of zinc-zireonium compound, to meet commercial standards.

Following alloying, or melt-ingot the alloy it previously prepared, its temperature is-brought into the range of 1200 to 1275 and held within this temperature range. While in this range of temperature, the melt isheld quiescent, without permitting temperature fluctuations ex? ceeding 50 or preferably 20 Fahrenheit degrees, for at least 40 minutes, and preferably for 1 hour, was to allow settling to the bottom or lower portions of the melt any particles, such as unalloyed zinc-zirconium compound, which will settle out under these conditions. ,If desired, the settling operation may be extended for as long as 2 hours or more if convenient.

After settling'the melt, the supernatant portion is preferably transferred to a clean heated steel ;cru cible and held therein at a temperature within the range of 1200 to 5 pre e b y nd a c ve o vfused sal ne :flux an example of which is given above. The duration of the holding operation depends upon the requirements for molten metal to be cast subsequently and is not critical. In fact the holding operation following settling may be m e i d s red D ring th h ld ng s p. f u ed th temperature of the melt is desirablykept from fluctuating more than 50 Fahrenheit degrees while maintainingthe temperature in the range of l200 to 1275 .F.

The so-treated molten alloy, that'is the settled or held metal, is delivered to the casting step, as for example by decanting, ladling, or pumping the supernatant portion of the melt into a suitable cast-ing mold, such as the mold of a continuouscasting machine to which reference has been made, to effect solidification, the metal while in the molten state being held at a temperature not in excess of 1275 F.

The solidified metal soeobtained exhibits desirably acceptable freedom from segregation of zinc-zirconium compound, thereby enhancing its usefulness especially in making extrudes and forgings.

In determining the presence of segregated metal, a slice about 1 /2 inches in thickness is cut transversely of an ingot not the cast metal, e-.g. a 12 inch diameter ingot, and

an X-ray photograph is taken perpendicularly to the slice. In the photograph so-obtained the segregated metal is made evident by more or less equiaxed shadows outlining the size of the segregated particles of the zinczirconium compound. Shadows so-observed having a diameter of A; inch or more are regarded as evidence the alloying pots, the settling temperature to that in the settling pots, the holding temperature to that in the holding and constant level pots, and the casting temperature to the temperature of the metal entering the continuous casting mold in the arrangement of pots and mold to which reference is above made.

Table A110 com osition,

b hlancg Mg Alloying Settling Holding Casting Melt No. temperatemp te mp Remarks ture Zn, Zr, F. Temp Min. percent percent F time Ex. 1. 5. 95 0.75 1, 400 1. 230 120 1, 230 1, 230 Clean metal. Blank 1. 5. 85 0. 66 1, 400 1. 400 120 1, 350 1, 340 Excessive segregation.

5. 9 0.61 1, 400 1. 230 120 1, 230 1, 230 Clean metal.

5. 58 0. 61 1,400 1, 400 120 1. 350 1. 380 Excessive segregation.

5. 37 0.67 1, 220 1, 220 90 1, 250 1. 250 Clean.

5. 0. 53 1, 220 1, 220 90 1, 370 1, 370 Excessive segregation.

5. 43 0. 6G 1, 350 1. 230 00 1, 265 1. 265 Clean.

5. 47 0. 65 1, 350 1, 350 90 1, 300 1, 300 Excessive segregation.

5. 52 0.67 1, 350 1, 240 G0 1. 300 1. 300 Excessive segregation. Ex. 6. 5. 92 0.65 1, 250 1, 220 60 1,210 1, 210 an. Blank 6. 5. 6 0.66 1. 350 1.350 60 1, 260 1,260 Excessive segregation. Ex. 7. 5. 9 0. 4 1, 250 1. 220 80 1, 220 1, 202 an. Blank 7. 5. 75 0.13 1, 400 1. 400 90 1. 350 1, 350 Excessive segregation. Ex. 8 5.4 0. 73 1, 220 1, 220 1, 275 1, 275 Clean.

1 Maximum fluctuation i111 Fahrenheit degrees.

of excessive segregation. By diameter is meant the diametrical dimension of each shadow of individual discrete particles, or closely packed aggregates of particles producing a single shadow, as seen in the X-ray photograph. Metal having a lesser amount of particle shadow is regarded as clean or not excessive as to the amount of segregation.

Although the invention may be used in providing a supply of the molten alloy for casting in sand or like molds probably its most useful application is in the continuous casting of ingot which by its nature requires a continuous supply of a large amount of the molten alloy in proper condition for casting without excessive segregation. In one installation of this type, two melting pots are provided in which charges of magnesium for making the alloy are alternately melted. Operated in conjunction with each such melting pot is a pair of alloying pots and one settling pot so that there are two groups of four pots in the arrangement. Each alloying pot of its pair is charged alternately with molten magnesium from the melting pot in conjunction with which it is operated and the alloying metals zinc and zirconium added as already described. The batches of alloyed metal so-obtained from one pair of the alloying pots are transferred alternately into its settling pot. Similarly the batches of alloy made in the other pair of alloying pots are transferred alternately to its settling pot. After permitting the alloy batches to settle in each settling pot the settled supernatant alloy is transferred first from one settling pot and then the other into a single holding pot which serves as a reservoir of molten alloy and from which prepared molten alloy may be withdrawn at a substantially constant rate. The flow rate from this pot is further smoothed out prior to feeding the continuous mold by transferring the molten alloy from the holding pot to another pot referred to as the constant level pot in which the level of the molten alloy therein is maintained substantially constant. From the constant level pot, the molten alloy is transferred at a substantially constant rate to the continuous casting mold.

The following tabulation of data of various examples of melts of magnesium-base alloy containing zinc and zirconium is illustrative of the practice of the invention and includes contrasting melts (blanks) treated at temperatures outside the ranges involved in the present inyention for comparison. In the tabulation, the alloying temperature corresponds to that of the molten alloy in From the table, it is manifest that in the tests the temperature at which settling and casting are effected, following alloying, is sharply critical for all alloying temperatures from 1220 to 1400 F. inclusive. In Example l, for instance, the temperature of settling, holding, and casting was maintained at 1230 F., following the alloying operation, thereby producing clean metal (freedom from excessive segregation of zinc-zirconium compound). This example may be contrasted with blanks 1, 2, 4, and 7 in which the settling, holding, and casting temperature was above 1275 F. Although there was not much change in temperature from the alloying to the casting steps in these blanks, yet the cast metal exhibited excessive segregation. In blanks 3 and 5, although the settling temperature is in the range in which clean metal can be had when cast cold enough, the cast metal obtained exhibited excessive segregation. However merely having the metal relatively cool for casting as in blank 6 does not yield clean metal with settling at 1350" F. It should be observed that in blank 7 the alloy contained but 0.13 percent of zirconium and the temperature of the molten alloy changed but little between alloying, settling, holding, and casting. Yet the cast metal exhibited excessive segregation. Sharply contrasting this result is Example 8 in which even though the zirconium content of the alloy was as high as 0.73 percent and the settling time was but 45 minutes, clean cast metal was obtained. Similar unusual results are exhibited in Examples 2 to 6, inclusive, showing the practice of the invention with variations in the amounts of the alloying constituents and variations of temperature of settling, holding, and casting within the limits of the invention.

This application is a continuation-in-part of our copendino application Serial No. 629,490, filed December 20, 1956, now abandoned. i

We claim:

1. The method of preparing molten magnesium-base magnesium-zinc-zirconium alloy containing from 1 to 9 percent of zinc and from 0.1 to 0.9 percent of zirconium in a condition for solidification without excessive segregation of zinc-zirconium compound which comprises alloying the zinc and zirconium with molten magnesium at a temperature between 1210 and 1450 F., settling the resulting melt at 1200 to 1275 F., keeping the metal during and after settling within the temperature range of 1200 to 12,75 R, the length of time the metal is in the temperature range of 1200 to 1275 F. before the following separating step being at least 40 minutes, and separating the supernatant settled metal from the remainder while maintaining the alloy at a temperature between 1200 and 1275 F., said separated supernatant metal so-obtained on being solidified by cooling from a temperature between 1220 and 1275 F. forming solid metal free from excessive segregation of zinczirconium compound.

2. The method according to claim 1 in which the separated supernatant metal is transferred at a temperature between 1200 and 1275 F. to a casting mold.

3. The method according to claim 2 in Which the separated supernatant metal is held in a quiescent state at a temperature between 1200 to 1275" F. prior to transfer to the casting mold.

4. The method according to claim 2 in which the temperature of the separated supernatant metal is not permitted to fluctuate more than 50 Fahrenheit degrees in the range of 1200 to 1275 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,042,684 Schjoth June 2, 1936 2,228,781 Sauerwald Jan. 14, 1941 2,235,508 Von Zeppelin et al Mar. 18, 1941 2,250,687 Von Zeppelin July 29, 1941 2,520,753 Ball et al. Aug. 28, 1950 2,546,931 Leman et a1. Mar. 27, 1951 2,664,353 Saunders et a1. Dec. 29, 1953 2,703,753 Nelson Mar. 8, 1955 2,786,755 Paddock et a1. Mar. 26, 1957 FOREIGN PATENTS 501,067 Belgium Feb. 5, 1951 749,168 Great Britain May 16, 1956 OTHER REFERENCES Modern Metals, January 1950, pages 26-28, article by Meier, Lib. 

1. THE METHOD OF PREPARING MOLTEN MAGNESIUM-BASE MAGNESIUM-ZINC-ZIRCONIUM ALLOY CONTAINING FROM 1 TO 9 PERCENT OF ZINC AND FROM 0.1 TO 0.9 PERCENT OF ZIRCONIUM IN A CONDITION FOR SOLIDIFICATION WITHOUT EXCESSIVE SEGREGATION OF ZINC-ZIRCONIUM COMPOUND WHICH COMPRISES ALLOYING THE ZINC AND ZIRCONIUM WITH MOLTEN MAGESIUM AT A TEMPERATURE BETWEEN 1210* AND 1450*F., SETTLING THE RESULTING MELT AT 1200* TO 1275* F., KEEPING THE METAL DURING AND AFTER SETTLING WITHIN THE TEMPERATURE RANGE OF 1200* TO 12,75*F. THE LENGTH OF TIME THE METAL IS IN THE TEMPERATURE RANGE OF 1200* TO 1275*F. BEFORE THE FOLLOWING SEPARATING STEP BEING AT LEAST 40 MINUTES, AND SEPARATING THE SUPERNATANT SETTLED METAL FROM THE REMAINDER WHILE MAINTAINING THE ALLOY AT A TEMPERATURE BETWEEN 1200* AND 1275*F., SAID SEPARATED SUPERNATANT METAL SO-OBTAINED ON BEING SOLIDIFIED BY COOLING FROM A TEMPERATURE BETWEEN 1220* AND 1275*F., FORMING SOLID METAL FREE FROM EXCESSIVE SEGREGATION OF ZINCZIRCONIUM COMPOUND. 